1. Field of the Invention
The present invention relates to an integral hydraulic transaxle (hereinafter, “IHT”), including a hydrostatic stepless transmission (hereinafter, “HST”) and an axle driven by the HST, improved in minimization, expansion of a neutral zone of the HST, and moderation of hydraulic pressure change.
2. Related Art
Conventionally, there is an assembly named as “IHT”, including an axle, a hydraulic pump and a hydraulic motor for driving the axle. The hydraulic pump and motor are fluidly connected to each other so as to constitute an HST. The IHT may include an axle housing supporting the axle therein. Each of the hydraulic pump and motor may be disposed in or out of the axle housing.
Japanese Patent Publication No. Hei 5-51487 discloses an example of the IHT, whose housing includes a center plate and a pair of left and right axle housing parts. The axle housing parts journaling respective axles are joined to respective left and right opposite side surfaces of the center plate so as to sandwich the center plate therebetween. The axles are disposed perpendicular to the center plate and rotatably supported at proximal ends thereof in the center plate. Also, the axles project laterally outward from the respective axle housing parts.
In the IHT, a hydraulic motor is disposed in one of the axle housing parts, and slidably rotatably fitted onto the side surface of the center plate so as to be fluidly connected to fluid ducts formed in the center plate. The hydraulic motor has a cylindrical motor shaft, which is coaxially and relatively rotatably penetrated by one of the axles and drivingly connected to the axles. The axle housing part, compactly containing the hydraulic motor with the motor shaft around the axle, is joined to the center plate so as to facilitate assembly of the IHT.
However, the IHT has the two joint surfaces, each of which is disposed between the center plate and each of the axle housing parts, to be oil-sealed. Further, a hydraulic pump is disposed in a pump housing separated from the left and right axle housing parts and the center plate. To fluidly connect the hydraulic pump to the hydraulic motor via the fluid ducts in the center plate, additional oil passages must be interposed between the center plate and the hydraulic pump, thereby preventing facility of the IHT in minimization, assembly and oil-sealing.
Additionally, a general IHT has a variable displacement hydraulic pump having a movable swash plate interlocking with a control arm pivoted by an IHT housing, so that rotation of the control arm tilts the swash plate for steplessly changing the displacement of the hydraulic pump, thereby changing the rotational direction and speed of an axle of the IHT. The control arm is biased toward its neutral position by a spring or the like, and interlockingly connected to a speed changing operation device such as a pedal or a lever.
In this regard, the IHT may have a system for controlling shock or jerk of a vehicle when the speed changing operation device is suddenly operated to start, stop or accelerate a vehicle having the IHT. For example, U.S. Pat. Nos. 5,836,159 and 6,109,032 discloses an IHT having valves for expanding a neutral zone of an HST of the IHT. The valves are slidably fitted into a center section in an IHT housing and fluidly connected to fluid ducts, which are formed in the center section for mutually fluidly connecting hydraulic pump and motor. A control arm interlockingly connected to a movable swash plate of the hydraulic pump has a surface, which is disposed along the center section so as to be pressed against the valves projecting from the center section. The surface of the control arm abutting against the valves is formed with grooves (or a groove) fluidly communicating with an oil sump in the IHT housing. When one or both of the contacting valves come to be open to the groove or grooves, oil flows out from the fluid duct or ducts in the center section to the oil sump. The groove is (or the grooves are) formed so that the open range of the groove (or grooves) to the valve (or valves) is disposed around a position of the control arm corresponding to the neutral position of the swash plate, thereby expanding the neutral zone of the HST, and moderating the motion of a vehicle having the IHT, even if the speed changing operation device is quickly operated for starting or stopping of the vehicle.
However, the IHT requires a member for adjusting a neutral position of the control arm to match with the neutral position of the swash plate. The member is an eccentric pin separated from the control arm, which requires an operator's labor of screwing a nut or a bolt. Further, a rotary axial shaft of the control arm is disposed in parallel to the sliding direction of the valves, so as to require a considerably large space for rotation of the control arm, thereby preventing minimization of the IHT.